Congenic-resistant strains of mice, possessing chromosomal segments from a high-responding strain on a low-responding background strain, were used to isolate the genes involved in determining the capacity of bone marrow-derived precursors of antibody-forming cells (B cells) to make an antibody response to Type III pneumococcal polysaccharide (SSS-III) and to assign such genes to well-defined genetic linkage groups of chromosomes. The results obtained show that, although at least five genes influence the capacity of B cells to make an antibody response to SSS-III, these genes are located on different (more than three) chromosomes.